Shortening of the wavelength of laser light and increasing of the numerical aperture NA of an objective lens can increase recording density and recording capacity in an optical disc. On the other hand, as the wavelength of the laser light is shorter and the numerical aperture of the objective lens is larger, there is a tendency that coma aberration is generated and reduces accuracy of recording and reproducing information. However, margin for tilt (warpage) of the optical disc can be ensured by making a light-transmitting layer of the optical disc thin, thus ensuring the accuracy of recording and reproducing information.
In recent years, an optical disc has attracted attention, in which blue-violet laser light having a wavelength as short as about 405 nm is used and numerical aperture NA is increased up to about 0.85 in order to largely increase the recording capacity. A light-transmitting layer having a thickness of about 0.1 mm that is thinner than a substrate having a thickness of about 1.1 mm is formed in accordance with the above wavelength and NA (see Japanese Patent Laid-Open Publication No. 2003-85836, for example).
This type of optical disc has high recording density and therefore the accuracy of recording and reproducing information is easily affected by dust, scar, or the like. Thus, a cartridge in the form of a thin box is prepared. The cartridge has an opening for transmitting information to/from the optical disc. The optical disc can be used in recording and/or reproduction equipment while being mounted on the cartridge. Moreover, another type of cartridge that has the opening for transmitting information and another opening for allowing printing to be preformed on the optical disc and/or allowing the optical disc to be removed and inserted therethrough.
In this type of optical disc, the numerical aperture NA of the objective lens is increased up to about 0.85 as described above, and a distance between the objective lens and the optical disc is shortened in accordance with the increased NA, as compared with a conventional optical disc such as CD (Compact Disc) and DVD (Digital Versatile Disc). Thus, there is a tendency that warpage causes interference between the optical disc and the objective lens more easily in this type of optical disc than in the conventional optical disc, even if the degree of the warpage is the same.
This type of optical disc is manufactured by forming a light-transmitting layer by spin coating on a substrate formed by injection molding, for example. Thus, the substrate and the light-transmitting layer are different in thickness and manufacturing method, and are sometimes different in material. Therefore, this type of optical disc has a problem that warpage can easily occur with a rapid change in a temperature or a humidity.
For example, when an optical disc that is placed in an outdoor location in winter and is therefore cooled down to a temperature close to an outside air temperature is mounted on a video camera or the like that is heated by being turned on, an environmental temperature of the optical disc increases by several tens of degrees C. in several seconds and warpage may occur. Moreover, when an optical disc that is placed in an outdoor location in summer and is therefore heated to a temperature close to an outside air temperature is mounted on a recording and reproduction device or the like that is placed and cooled in an air-conditioned room, an environmental temperature of the optical disc lowers by several tens of degrees C. in several seconds and warpage may occur. Large warpage can easily cause an error in reading and reproduction of information. Therefore, an angle of warpage should be suppressed within a predetermined limit value.
It is possible to suppress the angle of warpage within the predetermined limit value by making various improvements, for example, forming a layer having a coefficient of linear expansion equal to that of the light-transmitting layer on an opposite surface of the substrate to the light-transmitting layer.
However, even in the case where the warpage of the optical disc only is suppressed within the predetermined limit value, when that optical disc is mounted in the cartridge, warpage exceeding the predetermined limit value may occur in the optical disc in the cartridge or warpage of the cartridge may occur. That warpage may cause an error in reading and reproduction of information.
In order to develop a highly reliable optical recording medium in which the angle of warpage of the optical disc and the cartridge is suppressed, it is important to quantitatively understand the angle of warpage caused by a rapid temperature change or a rapid humidity change of the optical disc mounted in the cartridge or the cartridge that occurs in a short time.
However, in the conventional optical disc such as CD and DVD, the numerical aperture of the objective lens is small and there is a sufficient distance between the optical disc and the objective lens. Thus, warpage caused by a rapid temperature change or a rapid humidity change in a short time does not become a problem. Therefore, there is no method for measuring an angle of warpage of an optical recording medium caused by a rapid temperature change or a rapid humidity change occurring in a short time.
Moreover, there is no method for measuring an angle of warpage of an optical disc while the optical disc is mounted in a cartridge.